Optical path switching based differential absorption radiometry for substance detection

• US 6,611,329 B2
• Filed: 11/10/1999
• Issued: 08/26/2003
• Est. Priority Date: 04/20/1998
• Status: Expired due to Fees

• Current Assignee: United States Incorporated, United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Sachse, Glen W.
• Primary Examiner(s): Nguyen, Tu T.
• Application Number:
  US09/437,616
• Publication Number:
  US 20030112435A1
• Time in Patent Office: 1,385 Days
• Field of Search: 356/364-370, 356/300-317, 356/485, 356/414, 356/43, 356/73, 250/373, 250/338.5, 250/339.11, 250/339.13, 250/227.11, 250/231.19, 359/237-246
• US Class Current: 356/364
• CPC Class Codes: G01J 3/08 Beam switching arrangements
• CPC Class Codes: G01N 21/314 with comparison of measurements at specific and non-specific wavelengths dual wavelength spectrometry G01J3/427
• CPC Class Codes: G01N 21/3518 Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques

Abstract

An optical path switch divides sample path radiation into a time series of alternating first polarized components and second polarized components. The first polarized components are transmitted along a first optical path and the second polarized components along a second optical path. A first gasless optical filter train filters the first polarized components to isolate at least a first wavelength band thereby generating first filtered radiation. A second gasless optical filter train filters the second polarized components to isolate at least a second wavelength band thereby generating second filtered radiation. A beam combiner combines the first and second filtered radiation to form a combined beam of radiation. A detector is disposed to monitor magnitude of at least a portion of the combined beam alternately at the first wavelength band and the second wavelength band as an indication of the concentration of the substance in the sample path.

Claims

1. A method of detecting a substance, comprising the steps of:

receiving radiation passing along a sample path of interest;

dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;

routing said first polarized components along a first optical path and said second polarized components along a second optical path;

reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;

filtering said second polarized components to isolate at least a second wavelength band wherein second filtered radiation is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation; and

monitoring magnitude of said combined beam alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

2. A method according to claim 1 further comprising the step of performing gas filter correlation radiometry using one of said first polarized components and said first filtered radiation, and using one of said second polarized components and said second filtered radiation.

3. A method according to claim 1 wherein all wavelengths of said first polarized components are reflected except a plurality of unique first wavelength bands whereby said first filtered radiation excludes said plurality of first wavelength bands, and wherein said step of filtering said second polarized components comprises isolating a plurality of unique second wavelength bands wherein second filtered radiation is generated, said method further comprising the step of:

partitioning said combined beam into a plurality of partitioned beams, each of said plurality of partitioned beams comprising first filtered radiation and second filtered radiation, said first filtered radiation having excluded therefrom one of said plurality of unique first wavelength bands, wherein spectral absorption of a substance of interest is different at said one of said plurality of unique first wavelength bands as compared to one of said plurality of unique second wavelength bands,

wherein said step of monitoring comprises the step of monitoring magnitude of each of said plurality of partitioned beams.

4. A method according to claim 1 wherein a plurality of related substances of interest are spectrally absorbed more at said first wavelength band than at said second wavelength band.

5. A method according to claim 4 wherein said plurality of related substances are hydrocarbons.

6. A method according to claim 1 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

7. A method according to claim 1 wherein said step of monitoring is harmonically synchronized to a frequency at which said time series alternates between said first polarized components and said second polarized components.

8. A method of detecting a substance, comprising the steps of:

receiving radiation passing along a sample path of interest;

in dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;

routing said first polarized components along a first optical path and said second polarized components along a second optical path;

reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;

reflecting all wavelengths of said second polarized components except at least a second wavelength band wherein second filtered radiation excluding said second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation; and

monitoring magnitude of said combined beam alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

9. A method of detecting a substance, comprising the steps of:

receiving radiation passing along a sample path of interest;

dividing said radiation into a time series of alternating first polarized components and second polarized components orthogonal to said first polarized components;

routing said first polarized components along a first optical path and said second polarized components along a second optical path;

reflecting all wavelengths of said first polarized components except at least a first wavelength band wherein first filtered radiation excluding said at least first wavelength band is generated;

reflecting all wavelengths of said second polarized components except at least a second wavelength band wherein second filtered radiation excluding said second wavelength band is generated, wherein said first wavelength band and said second wavelength band are unique and wherein spectral absorption of a substance of interest is different at said first wavelength band as compared to said second wavelength band;

combining said first filtered radiation and said second filtered radiation to form a combined beam of radiation;

transmitting the combined beam to a filter having a band pass characteristic that spans said excluded first and second wavelength bands; and

monitoring magnitude of said combined beam received from the filter alternately at said first wavelength band and said second wavelength band as an indication of the concentration of said substance in said sample path.

10. A method according to claim 9 further comprising the step of performing gas filter correlation radiometry using one of said first polarized components and said first filtered radiation, and using one of said second polarized components and said second filtered radiation.

11. A method according to claim 9 wherein all wavelengths of said first polarized components are reflected except a plurality of unique first wavelength bands whereby said first filtered radiation excludes said plurality of first wavelength bands, and wherein all wavelengths of said second polarized components are reflected except a plurality of unique second wavelength bands whereby said second filtered radiation excludes said plurality of second wavelength bands, said method further comprising the step of:

partitioning said combined beam into a plurality of partitioned beams, each of said plurality of partitioned beams having excluded therefrom one of said plurality of unique first wavelength bands and one of said plurality of unique second wavelength bands, wherein spectral absorption of a substance of interest is different at said one of said plurality of unique first wavelength bands as compared to one of said plurality of unique second wavelength bands,

wherein said step of monitoring comprises the step of monitoring magnitude of each of said plurality of partitioned beams.

12. A method according to claim 9 wherein a plurality of related substances of interest are spectrally absorbed more at said first wavelength band than at said second wavelength band.

13. A method according to claim 12 wherein said plurality of related substances are hydrocarbons.

14. A method according to claim 9 wherein said first polarized components are vertically polarized and said second polarized components are horizontally polarized.

15. A method according to claim 9 wherein said step of monitoring is harmonically synchronized to a frequency at which said time series alternates between said first polarized components and said second polarized components.

16. A method according to claim 9 wherein said combined beam of radiation comprises a time series that alternates between said first polarized components and said second polarized components.